SAE Flareless Bite Type Fittings (Parker Ferulok®)Ferulok Overview
Design criteria require any fitting to achieve two functions – it must both seal against leaks and mechanically hold against system pressure. Unlike flare fittings which achieve both functions at the flare, flareless fittings are more complex.
Sealing must be achieved at a minimum of two points (more in the case of two ferrules). First, a line contact seal must be "coined" between the contact point of the ferrule and the internal body seat. Second, a "bite" seal deep enough to penetrate surface scratches on the tube, must be established between the ferrule and the tube.
Mechanical holding power is achieved by the "bite" into the tube causing displaced metal to be pushed up into a "wall" ahead of the bite. This "wall" prevents the tube from blowing out. The amount of force that is acting to push the tube out is calculated by multiplying system pressure by the cross section area of the tube. For example, blowout force on a 1/2" OD tube at 1000 psi static pressure is 197 lbs. Note that in the SAE flareless fitting, the bite is just as effective on a solid bar as it is on the medium to heavy wall tubing for which it was designed. Compare this to light duty instrumentation fittings (one or two ferrule) which rely on mechanical swaging of the tube for holding power. In these fittings, as the wall thickness increases, less swaging action is possible making such fittings unsuitable for heavy wall tubing and high pressures.
Vibration must be totally isolated from the "bite" area in any flareless fitting. The "bite" creates a stress riser which will cause fatigue and cracking if the vibration is not controlled. The SAE flareless fitting with its angle at the rear of the ferrule causes the ferrule to "bow" into a barrel shape and grip the tube at the rear. The rear grip channels vibration into the nut/body system shielding the "bite” area. Examination of two ferrule instrumentation fittings reveals that tube vibration is fed directly into the "bite" making these fittings undesirable for high pressure, high vibration service.
Remakeability & Reusability
All flareless fittings must be tightened a bit more upon each remake. SAE hydraulic flareless fittings are designed for 15 remakes. Reusability of parts is limited in that a new ferrule is needed to re-tube.
While there is no need in small sizes, in 5/8” OD and larger all manufacturers recommend mechanically presetting the ferrule. The fitting is not recommended for thin wall tubing because of the high forces exerted upon the tube by the ferrule.
Because the dimensions of the fitting are controlled by the Society of Automotive Engineers (SAE), all manufacturers’ fittings will interchange, piece by piece.
Only Parker has these important features: an exposed "bite" allowing visual inspection; forged shapes and nuts; rolled external threads; more shapes and sizes and availability in steel, stainless and monel.
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